The present invention relates generally to methods and circuits for sensing output current and voltage in a floating output power supply.
Sensing output current and voltage sensing in a power supply with an output referenced to a ground of the input power source is relatively simple. A current sensing resistor configured in series with the output (e.g., coupled between the ground and the positive output of the power supply) is used to sense current, and the voltage of the output can be directly sensed or reduced via a resistive network. However, the output of a floating output power supply is not referenced to the ground of the input power source. Direct output voltage and current sensing is therefore more complicated.
Referring to prior art FIG. 1, a floating output power supply has a positive output 102 and a negative output 104. However, the negative output 104 is not tied to a ground of the input power source 106 and/or controller 108. In one embodiment, the input power source 106 is a voltage controlled variable frequency alternating current (AC) voltage source. The controller 108 is referenced to a ground of the input power source 106 and varies the frequency of the input power source as a function of a sensed voltage (i.e., V_sense) and/or sensed current (i.e., I_sense).
Operational amplifiers provide differential input sensing characteristics that are conventionally used to sense the output voltage and output current, and to provide signals indicative of the sensed output voltage (i.e., V_sense) and current (i.e., I_sense) to the controller 108. Operational amplifier based differential sensing solutions are typically expensive and complicated due to wide fluctuations in the voltage of the negative output relative to the ground of the input power source 106 and controller 108.